Ferrous-based sintered materials have typically not been a material of choice for utilization in high strength applications because of their intrinsic porosity. However, sintered products have high versatility as to shape and are easily manipulated into complex forms for relatively low cost. In general, sintered products exhibit low strength when formed from low carbon materials, and they have low formability when containing a significant amount of carbon.
In the case of low carbon materials, surface densification is a technology that greatly improves mechanical properties and strength, in particular contact and bending fatigue properties. This technology has been proven efficient for manufacturing moderately loaded automotive powertrain components such as gears, sprockets and races using low or carbon free materials. Several methods have been proposed to surface densify these articles. Among them, several have been successfully implemented in high volume manufacturing, being those that perform the surface densification as some form of a cold forming process. Cold forming, as compared to hot forming, has four key advantages: (1) results in high precision components; (2) has low tooling wear; (3) avoids oxidation of the work piece; and (4) does not require heating of the work piece. However, cold forming has important limitations as well. It is well known in metal forming technology that cold forming is severely limited by: (1) high yield strength of the material that results in extremely high forming stress that, in turn, can induce tooling failure and requires larger forming equipment; and (2) low material workability, in that the material ability to withstand plastic strain, without failure, e.g., fracture, under a given stress state is reduced. These deleterious effects are exacerbated by the presence of carbon in the powder mixture in any significant amount. While higher carbon concentrations add strength to the component parts, these higher carbon components are only effective if little or no post sintering workability is required.
The presence of carbon, furthermore, does not facilitate an increase in density of the sintered forms. In most applications, full densification of the surface layer is required for reasonable performance, which is generally incompatible with alloys containing a significant amount of carbon.
The workability or formability of the material is further limited by the presence of porosity, which greatly reduces the strain required to cause fracture. For example, sintered steel with more than 0.3 wt % carbon and 5% or more porosity is limited to 0.5-2% deformation before rupture. For an effective densification, a component with a density of 7.2 g/cm3 will need over 9% deformation to reach a full density of 7.87 g/cm3. The required level of deformation will be higher if the initial density of the part is lower.
As stated above, surface densification methods have primarily relied on the use of materials with either no or very low carbon content, typically under 0.2 wt %. Iron (no carbon) or low carbon steels exhibit workability that is higher than that of higher carbon steel. However, the use of low carbon steel has some important practical limitations. Low carbon steels are not directly heat treatable; they require addition of carbon prior to heat treatment, typically through a gas carburization process. Carburizing processes are lengthy and expensive, particularly for large components. Such heat treatment produces a hard surface layer of few millimeters (0.1 to 2 mm) deep and a relatively carbon free soft core. In some applications where the stresses are confined to the shallow surface layer, the current technology has produced very good results. Indeed, careful selection of alloying elements and optimization of post-sintering operations such as the carburization process have been successfully applied in manufacturing of high performance surface densified low carbon powder metal products. As reported in SAE Paper # 0396, 2006 by Trasorras, Nigarura and Sigl and in Gear Solutions pages 18-22, July 2006 by Ulf Engstrom, surface densification of low carbon powder metal components produces equivalent or higher performance when compared with low carbon wrought or forged steels.
These successful developments in powder metal technology, however, fail to provide the performance required in applications where through-hardened forged components are necessary, such as those disclosed in U.S. Pat. Nos. 3,992,763 and 4,002,471.
An alternative method to produce surface densified high carbon materials which are competitive with through-hardened forged components is therefore lacking in the art. Such a method would need to be capable of producing a surface densified product which can be hardened directly in a sintering furnace if given an accelerated cooling; hardened by induction directly after sintering or through-hardened by a short austenitization period followed by oil quenching.